As the steady cadence of turning pedals carried Michelle Harburg through the spitting morning rain, cars quickly stacked up behind her in the right lane of Constitution Avenue near the shadow of the Capitol.

Caught in the first surge of the day's rush hour, the driver of a white Acura tried to inch past her, only to fall back as a Metrobus loomed just to the left. A second try at a lane change ran afoul, with a black Cadillac SUV bearing down. A horn sounded, brake lights flashed and then the Acura slipped by.

"About 60 percent of the drivers are positive," Harburg said as she locked her bike a few minutes later. "And 40 percent are negative, and when I say negative, mostly I mean just not aware. About 10 percent of them just don't like bikers. They shout at you, 'Get off the Road!' or beep just to try to scare you to death."

This week is Bike to Work Week, an annual event devoted to the notion that commuters ought to switch from four wheels to two. This year, it comes as miles of bike lanes are being added in the District and throughout the region and the nation. Thousands of Washington residents are expected to make the switch Friday for Bike to Work Day.

The challenge of merging bikes and cars on highways has had consequences: The overall number of traffic fatalities in the United States has dropped to its lowest point since 1961, but deaths of bike riders are up.

Sometimes, there are consequences for drivers and cyclists alike. Royal Kessick of Richmond was indicted on manslaughter charges three weeks ago by a St. Mary's County grand jury that looked into his fatal collision with cyclist Hugo Gonzalez last summer. Baltimore County authorities are investigating Faith Frenzel's crash last month that killed Larry Bensky and badly injured his cycling buddy, Joel Wyman.

Bensky and Gonzalez, athletes struck down while training, were unusual victims, a review of the region's cycling fatalities suggests.

"The people who do it for sport generally handle their bikes pretty well," said Detective Scott Neville, who investigates fatal accidents in Fairfax County. "It's the people who are biking to and from work, without so much awareness or understanding of the law, who get into trouble."

A new role for bikes

America's much-chronicled love affair with the automobile and the beckoning freedom of the open road are experiencing something of a midlife crisis. Bicyclists just add one more variable to the equation.

Frustrated by congested traffic, busy with cellphones and often in a rush, drivers are displaying less patience than ever with cyclists who block traffic and sometimes ride erratically, flying through stop signs and traffic lights.

For bike riders, the dangers of sharing roads range from honks, shouts and thrown objects to distracted or drunk drivers and those who try to crowd bicyclists from the road.

"I realize that we can be our own worst enemies when we blow through stop signs," said Andy Clarke, president of the League of American Bicyclists. "Clearly, people are irritated by our presence, but to the point of wishing to hit us or brush up against us? It blows my mind."